1. Field of the Invention:
The present invention relates to an apparatus for conveying a sheet by member by means of suction-type rollers. More particularly, the present invention is concerned with an apparatus which is capable of conveying a sheet member at a constant speed and with a high degree of precision and which is suitable for use in a system for scanning the sheet material by a light beam.
2. Description of the Prior Art:
Japanese Patent Laid-Open No. 33451/1984 discloses an apparatus for conveying a sheet member which is to be scanned by a light beam. This apparatus employs a porous endless belt and a suction box disposed inside the endless belt. The vacuum generated in the suction box is transmitted through the pores in the porous endless belt so as to attract the sheet member, whereby the sheet member is conveyed by the porous endless belt while keeping close contact with the latter. With this arrangement, therefore, it is possible to convey the sheet member at a constant speed and with a high degree of positional precision without any offset of the sheet member, while keeping a high degree of flatness of the sheet member surface to be scanned.
This apparatus also employs a detection means for detecting the leading end of the sheet member to be scanned, and the scanning system for scanning the surface of the sheet member is controlled in accordance with the output from this detection means, so that the whole area of the sheet member can be scanned or exposed correctly with high positional accuracy.
This known apparatus, however, suffers from the following disadvantages. Firstly, it is to be pointed out that the apparatus essentially requires an expensive and space-occupying suction system, as well as expensive crown pulleys for the endless belt. In addition, it is necessary to employ a motor having a large driving torque for driving the endless belt, considering that the load on the motor is largely varied depending on the positional relationship between the suction box and the sheet member carried by the endless belt.
Another known apparatus for conveying a sheet member to be scanned by a light beam employs a suction roller in place of the combination of the suction box and the endless belt mentioned above. In operation, the sheet member is sucked onto the suction roller and conveyed in accordance with the rotation of the suction roller. This known apparatus also encounters a problem in that, since the suction roller is hollow and provided with suction holes over the entire surface thereof, air is sucked also through the holes which are not covered by the sheet material on the suction roller, resulting in a wasteful use of the suction power and a low efficiency of the suction of the sheet member.